The electrical conductivity of many materials is variable as of function of temperature. Such materials, when packaged as temperature sensitive resistors, are called "thermistors" and are much used to measure temperature. The current that flows through the thermistor and the potential drop across the device is the analog of temperature.
Current and voltage changes in and across thermistors are faithfully repeatable. Thus, thermistors per se are accurate measuring devices. However, the accuracy that is achieved in practice depends upon ability to subject the thermistor to the temperature to be measured and to shield it from influences that operate to alter thermistor temperature from the test temperature value.
In practical applications it is necessary to mount the sensor and to enclose and shield it. The combination of the temperature sensing element and its housing or mounting is often called a temperature probe. Its mass and the thermal conductivity of its parts may differ greatly from the mass and conductivity of the object or medium whose temperature is to be measured.
In one important application measuring the temperature of air and other respiratory gases with a thermistor is complicated by the difference between the mass and thermal conductivity of the gas and the probe materials, and by the need to construct the probe so that it can be kept or made sterile for each use. The requirement for sterility usually dictates that the probe be removable from the respiratory apparatus and that the thermistor, or at least the lead wires to the thermistor, be enclosed in a housing or covering that is readily made sterile. Probes of that kind exist but their response, at least when measuring gas temperature, is unduly slow.
The need for fast, accurate respiratory gas temperature measurement is greatest when the gas temperature is elevated before delivery to the patient and when the patient is incapable of self-help or of sounding an alarm if the gas temperature control system fails. In an incubator for premature infants, for example, the respiratory gas is heated and humidified before delivery. A failure of the humidity or the temperature control system can result in a rapid change in gas temperature to a dangerous level. The infant is helpless to save itself and the temperature change is not visible or otherwise apparent to the attendant. The temperature sensing probe must respond immediately to temperature change with an appropriate signal. Neglecting other elements of the probe, the thermistor lead wires alone are capable of conducting heat away from the thermistor more efficiently than air and other gas mixtures can transfer heat to the thermistor, and the heat storage capacity of the lead wires is greater than that of air. The result is lagging response to temperature change and failure to achieve temperature of the thermistor equal to the surrounding air stream, i.e., it reads low. Finding a solution to that problem is complicated by the need to enclose the lead wires for the sake of cleanliness and strength. The enclosure must be accounted for in controlling heat transfer and storage.